[refurb]

A better question is why telephone quality is bad in general. I assume it's a bandwidth issue related to the technological limitation when it was rolled out.

Whenever I do a FaceTime voice call I'm always shocked by the quality of the audio.

[Slansitartop]

IIRC, the cell phone voice codecs are at an absurdly low bitrate (1.2kbps?) and highly optimized for human voice only, so voice is acceptable and everything else sounds like trash.

Analog landlines used 64kbps codecs (after the analog loop to your house) that were flexible enough to be abused to transport 56kbps of data.

I'm pretty sure nothing else lower quality codecs than cell phone voice.